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Plate Boundaries

It is apparent that most of what we observe on the surface of the earth and at shallow depth is related in some way to plate tectonics. The beaches along the eastern margin of North America are the result of the weathering, erosion, and breakdown of a huge mountain range (the Appalachian Mountains) over the last 300 million years. The Appalachians were formed during the collision of continents to form a mega continent called Pangaea. The volcanic mountains that form the western margin of North America are the result of subduction of oceanic crust under continental crust. The subduction results is frictional heating and partial melting of material being dragged down into the mantle.

Practically all of the major processes that occur on the surface of the earth are related to plate tectonics. Earthquakes, volcanism, and mountain building are three of the most spectacular earth processes related to tectonic activity, however many other processes like metamorphism, sedimentary rock formation, among others are related directly or indirectly to plate tectonics. In the interactive image below, you can see the relationship between volcanism, earthquakes and mountain building and the interaction of the tectonic plates.

There are three types of plate boundaries. They are divergent, convergent and oblique slip (transform). Each boundary type has its own characteristic geologic features and processes, by which it can be identified even millions of years after it has been active. It is clear that the area around the Appalachian mountains was once a subduction zone even though it has not been active for the last 250 million years.

Divergent boundaries are characterized by a rift in the surface of the earth along the midocean ridges that exist in all of the major ocean basins. The ridges are composed of volcanic mountains that erupt basalt onto the surface of the ocean and by that process create new oceanic crust. The almost constant eruption of basalt causes each side of the ridge to cool, crystallize, and move away from the ridge. Each pulse of basalt eruption causes the two sides of the rift zone to move a little farther apart. The rate of plate creation at divergent boundaries varies between about 2 and 6 centimeters per year. The creation of oceanic crust and movement away from the midocean ridges is called seafloor spreading. The most common processes occurring along divergent boundaries are volcanism and earthquakes. Many divergent boundaries are formed when a continent rifts apart and a midocean ridge forms between the two halves of the continent. Fault bounded rift valleys form at the edges of the continents that fill rapidly with relatively immature sediment and form rocks like arkosic sandstone and shale. These deposits, called red beds, are characteristic of the continental margins in divergent boundaries.

Convergent boundaries are boundaries at which crust is destroyed by the subduction of oceanic crust beneath continental crust or other oceanic crust. Convergent boundaries contain the broadest array of geologic activities. The movement of oceanic crust beneath another crustal plate results in heating of materials that are dragged down with the plate and eventual partial melting of the material. The resulting magma moves upward towards the surface of the plate boundary forming intrusions or andesitic volcanic eruptions. Contact metamorphism occurs along the boundaries of the magma bodies and intrusions. The convergence of plates also results in increased pressure and stress that, along with burial, result in regional metamorphism throughout the continental side of the convergent boundary. The grade of metamorphism increases with depth in the crust. Regional metamorphism also occurs in convergent boundaries in which oceanic crust is subducted under other oceanic crust, but it occupies a smaller volume and tends to be higher pressure and lower temperature, often called blueschist metamorphism. Mountain building accompanied by folding and faulting is another important feature of convergent boundaries. The folding is a mechanism of regional metamorphism at depth and the faulting results in localized dynamic metamorphism nearer the surface. It is important to remember that the rate of crustal destruction at convergent boundaries, around the world, is equivalent to the rate of crustal creation at divergent boundaries.

Transform or oblique slip boundaries are somewhat less common than the other boundaries and are characterized by the lack of crustal creation or destruction. At transform boundaries plates move by each other in the horizontal plane along transform and transcurrent fault systems. The two major transform boundaries in the world are the southwestern margin of the North American plate, along the San Andreas fault system, and the western margin of the Indian subcontinent and Arabian plate, manifested by major fault systems in central Asia. The major process that occurs at transform boundaries is transcurrent and transform faulting with the accompanying dynamic metamorphism. Transform boundaries are often characterized by some of the highest magnitude earthquakes recorded throughout history. This is the result of stress building up along asperities, or irregular zones of friction, along the fault that lock up and then break down when the stress along the boundary exceeds the strength of the asperity.

Occasionally in convergent boundaries, two continental land masses on the opposing plates end up colliding when all of the oceanic crust is subducted. When this occurs, tremendous mountain building occurs along with regional metamorphism, igneous activity and faulting. The largest mountain range in the world, the Himalayas, was formed when the indian subcontinent collided with the Eurasian plate.

It is clear that each of the three plate boundaries has its own set of characteristic structural features and processes that occur within them. In the exercise linked below you will have the opportunity to test your understanding of the features and processes at plate boundaries.